플라즈마-촉매 환원공정에서 촉매의 특성이 질소산화물 저감에 미치는 영향

Abstract

This study investigated the shape and size effects of supported metal oxide catalysts on the catalytic and plasma-catalytic reduction of nitrogen oxides(NOx)which cause various adverse effects on both environment and human health such as optical smog, acid rain, ground ozone for mation, chest pain, and pulmonary edema symptoms. In the first part, conventional hydrocarbon-selective catalytic reduction of NOx overγ-alumina supported metal oxides(M/γ-Al2O3, M: Ag, Cu and Ru) was studied in a reaction temperature range of 250 to 400oC. For Ag/γ-Al2O3catalyst, the NOx reduction efficiency increased as the size of Ag decreased, following the order: 20 nm>50 nm>80nm. In addition, the shape effect of catalysts on the NOx reduction was examined with spherical-and wire-shape nanoparticles. A higher catalytic activity for NOx reduction was observed with Ag and Cu wires than with the spheres, while spherical-and wire-shape Ru exhibited similar NOx reduction efficiency to each other. Among the metal oxides examined, the best catalytic activity for NOx reduction was obtained with Ag wire, showing almost complete NOx removal at a temperature of 300oC.For Cu and Ru catalysts, considerable amounts of NO was oxidized to NO2, rather than reduced to N2,leading to lower NOx reduction efficiency. In the second part of this study, a hybrid plasma-catalyst system for NOx reduction was investigated. Effects of various parameters including concentrations of hydrocarbon and oxygen, humidity, reaction temperature and plasma specific input energy (SIE) on the NOx conversion were examined. At a reaction temperature of 250oC and a SIE of 42 J/L, 83% NOx conversion was achieved with the wire catalyst, whilst that obtained with the spherical Ag was considerably lower with 69% under the same conditions. In presence of plasma, the reactor performance was greatly enhanced compared to that of the conventional thermal catalysis which contributed only 31% to NOx conversion (on wire Ag) at 250oC. It is deduced that the oxidation of NO to NO2was first facilitated in plasma by oxidizing species such as atomic oxygen and ozone followed by the selective catalytic reduction of NO and NO2mixture to N2by n-heptane and plasma-induced hydrocarbon intermediates. By increasing the n-heptane concentration, the NOx conversion was observed to increase with increasing the C1/NOx ratio up to a value of 5 and saturated thereafter. Meanwhile, the reactor performance increased with increasing the oxygen content from 3 to 15%, especially at low values of SIE. In contrast, humidity negatively affected the NOx conversion, which was lower at higher H2O vapor content.